1. Technical Field
This invention relates to an organic electroluminescent (EL) device and more particularly, to an inorganic/organic junction structure suitable for use in a device of the type wherein an electric field is applied to a thin film of an organic compound to emit light.
2. Background Art
In general, organic EL devices have a basic configuration including a glass substrate, a transparent electrode of ITO etc., a hole transporting layer of an organic amine compound, an organic light emitting layer of a material exhibiting electronic conductivity and intense light emission such as Alq3, and an electrode of a low work function metal such as MgAg, wherein the layers are stacked on the substrate in the described order.
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. The organic compound layers are typically of two- or three-layer structure.
Included in the two-layer structure are a structure having a hole transporting layer and a light emitting layer formed between the hole injecting electrode and the electron injecting electrode and another structure having a light emitting layer and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Included in the three-layer structure is a structure having a hole transporting layer, a light emitting layer, and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Also known is a one-layer structure wherein a single layer playing all the roles is formed from a polymer or a mixed system.
FIGS. 2 and 3 illustrate typical configurations of organic EL devices.
In FIG. 2, a hole transporting layer 14 and a light emitting layer 15 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11. In this configuration, the light emitting layer 15 also serves as an electron transporting layer.
In FIG. 3, a hole transporting layer 14, a light emitting layer 15, and an electron transporting layer 16 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11.
Reliability is a common problem to be solved for these organic EL devices. More particularly, organic EL devices in principle have a hole injecting electrode and an electron injecting electrode and need organic layers for effectively injecting and transporting holes and electrons from the electrodes, respectively. However, the materials of which the organic layers are formed are vulnerable during manufacture and have less affinity to the electrodes. Since the electron injecting electrode for injecting electrons must be formed of a metal having a low work function, such materials as MgAg and AlLi must be used. These materials, however, are readily oxidizable and unstable, becoming the serious factor that governs the life of organic EL devices and gives rise to a reliability problem. Another problem is raised by the significantly accelerated degradation of organic thin films as compared with light-emitting diodes (LED) and laser diodes (LD).
Also, most organic materials are relatively expensive. Any partial replacement of constituent films by an inexpensive inorganic material would give an economical merit in the manufacture of cost effective organic EL device-applied products.
There is also a desire to develop a device having a further improved luminous efficiency, a lower drive voltage and a less current consumption.
To solve this and other problems, a means for taking advantage of both an organic material and an inorganic semiconductor material has been devised. That is, an organic/inorganic semiconductor junction is established by substituting an inorganic p-type semiconductor for the organic hole transporting layer. These efforts are found in Japanese Patent No. 2636341, JP-A 139893/1990, 207488/1990, and 119973/1994. It was impossible to design EL devices which are superior in light emission properties and reliability to prior art organic EL devices.